1. Field of the Invention
The present invention relates to a conductive bond that is applied to a bump provided on a substrate when two substrates, each having a conductive pattern, are laminated. The present invention also relates to a multilayer printed circuit board including substrates that are laminated with this conductive bond and a method for making the multilayer printed circuit board.
2. Description of the Related Art
One of the methods for making multilayer printed circuit boards is a build-up process including the steps of forming a conductive pattern on an internal substrate, forming an insulating layer on the conductive pattern, and repeating these steps to form a plurality of conductive layers. If any layer has a defect during the above steps in the build-up process, the resulting multilayer printed circuit board is useless. Thus, this process has low productivity. A countermeasure to overcome such a problem is a method for making a multilayer printed circuit board including independently forming a plurality of substrate, each having a conductive pattern, laminating these substrate with insulating layers provided therebetween, and bonding them.
In this method, a conductive paste is used for tight electrical connection between the layers. Since the conductive paste contains, for example, copper particles with a particle diameter of about 10 xcexcm, and an epoxy resin binder, and a curing agent, the resistance at the connections may be unstable or high in some cases.
Another means for electrical connection between layers includes application of tin-lead solder to a connecting portion of one substrate. Since a flux serves as an impurity for the tin-lead solder, the connection must be performed without using the flux. Thus, the connecting portion does not have satisfactory adhesiveness. Furthermore, the tin-lead solder having a low melting point melts and expands during a reflow process for mounting electronic devices; hence, disconnection between the layers may occur during this step.
Another method for electrical interlayer connection includes bonding laminated substrates with gold as a bond by a thermal contact bonding process. Since the substrates are bonded by resin shrinkage in this method, the bonded portion is unreliable.
An object of the present invention is to provide a conductive bond that can electrically bond a plurality of substrates by thermal contact bonding at a temperature which is lower than that in known methods.
A further object of the present invention is to provide a conductive bond that can tightly bond a plurality of layers even if the heights of bumps are not uniform.
A still further object of the present invention is to provide a conductive bond that enhances productivity of multilayer printed circuit boards and reliability of connections.
Another object of the present invention is to provide a multilayer printed circuit board of which the layers are bonded with the above conductive bond.
Another object of the present invention is to provide a method for making the multilayer printed circuit board.
A conductive bond according to the present invention comprises conductive colloidal particles and a dispersant for dispersing the conductive colloidal particles. For example, the conductive colloidal particles are composed of silver, gold, and copper. Preferably, the conductive bond further comprises adhesive particles that enhance bonding strength between the two substrates. Preferably, the conductive bond further comprises heating particles that accelerate heating of the conductive bond. Preferably, the adhesive particles have an average particle size in the range of 10 to 100 xcexcm.
The conductive colloidal particles have high activity and a low melting point, which can decrease a processing temperature during a bonding operation and enhance processing efficiency. The conductive bond contains a reduced amount of the resin component compared with known conductive paste; hence, a connection formed by using this conductive bond has low resistance.
A multilayer printed circuit board according to the present invention includes a plurality of substrates, each having a conductive pattern on at least one face thereof, any adjacent two of the plurality of substrates being separated by an insulating layer, wherein the conductive pattern of a first substrate of the two substrates faces the conductive pattern of a second substrate of the two substrate. The conductive pattern of a first substrate has at least one bump for electrical connection to the second substrate. The bump and the conductive pattern of the second substrate are bonded to each other with a conductive bond applied to the tip of the bump. The conductive bond comprises conductive colloidal particles and a dispersant for dispersing the conductive colloidal particles.
Since the conductive bond is used for the connections between the bumps and the lands of the multilayer printed circuit board, the multilayer printed circuit board can be produced with high efficiency and the connections have low electrical resistance.
A method for making a multilayer printed circuit board according to the present invention comprises the steps of: forming at least one bump on a conductive layer on at least one face of a first substrate; patterning the conductive layer to form a conductive pattern; patterning a conductive layer on at least one face of a second substrate; applying a conductive bond to the tip of the bump; laminating the first substrate and the second substrate with an insulating layer provided therebetween such that the conductive pattern of the first substrate faces the conductive pattern of the second substrate; and electrically connecting the opposing conductive patterns through the bump by thermal contact bonding, wherein the conductive bond comprises conductive colloidal particles and a dispersant for dispersing the conductive colloidal particles.